1. Field of the Invention
This invention relates to urea-formaldehyde polymers having reduced formaldehyde emissions, to urea-formaldehyde cellular foams incorporating a foaming agent which inhibits acid-induced hydrolysis of the foam and to methods of making each of the above.
2. Prior Art Relating to the Disclosure
Urea formaldehyde resins are generally prepared by the acid catalyzed reaction of urea and formaldehyde. Such urea-formaldehyde resins contain amide linkages which are capable of hydrolysis and are therefore intrinsically unstable in the presence of water. Such hydrolysis generates low molecular weight material which is believed to liberate formaldehyde. It is also known that the inherent residual acidity in urea-formaldehyde polymers prepared using an acid catalyst is a critical factor in the hydrolytic breakdown of such polymers. The emission of formaldehyde from urea-formaldehyde resins and products incorporating urea-formaldehyde resins is and continues to be a problem. Numerous methods have been proposed to reduce the formaldehyde content of such resins, such as by use of one or more aryl dialdehydes having 2-8 carbon atoms provided as a cross-linking solution at the time of final polymer formation (U.S. Pat. No. 4,129,533). Also disclosed are calcium lignosulfonates, furfuryl alcohol, resorcinol and the addition of free urea to the catalyst solution during manufacture of urea-formaldehyde foams.
Urea-formaldehyde resins are widely employed in wood products, for example, in the manufacture of interior plywood and particleboard. Such resins are also widely used in the form of foams for insulation purposes. For such use urea-formaldehyde resins are formulated to produce a rapidly setting, low density foam having a low shrinkage made by mixing together two main components with aeration, the components including (1) an aqueous solution of a urea formaldehyde resin prepolymer and (2) an aqueous solution of a surfactant and a strong acid catalyst. Resin foams prepared as described are injected into the walls of dwelling units with specially designed equipment where they cure in place. The resins rapidly gel when placed in contact with acidic catalyst to produce highly cross-linked insoluble polymer. Typically, the urea-formaldehyde resins have a molar ratio of formaldehyde to urea ranging from 1.0:1.0 to 2.5:1.0, preferably 1.0 to 1.3:1.6. There is generally some unreacted monomer in the finally cured urea-formaldehyde resin, this monomer present as free formaldehyde or an N-methylol moiety at the polymer chain ends. During the extreme varying conditions of temperature and humidity to which such foams are exposed when used for insulation purposes in dwelling units, the formaldehyde is emitted as a vapor and can cause objectionable odors.